The manufacture of integrated circuits involves the transfer of geometric shapes on a mask to the surface of a semiconductor wafer. Thereafter, the semiconductor wafer corresponding to the geometric shapes, or corresponding to the areas between the geometric shapes, is etched away. The transfer of the shapes from the mask to the semiconductor wafer typically involves a lithographic process. This includes applying a solution of a pre-polymer solution to the semiconductor wafer, the pre-polymer being selected to form a radiation-sensitive polymer which reacts when exposed to ultraviolet light, electron beams, x-rays, or ion beams, for example. The solvent in the pre-polymer solution is removed by evaporation, and the resulting polymer film is then baked. The film is exposed to radiation, for example, ultraviolet light, through a photomask supporting the desired geometric patterns. The images in the photosensitive material are then developed by soaking the wafer in a developing solution. The exposed or unexposed areas are removed in the developing process, depending on the nature of the radiation-sensitive material. Thereafter, the wafer is placed in an etching environment which etches away the areas not protected by the radiation-sensitive material. Due to their resistance to the etching process, the radiation sensitive-materials are also known as photoresists, and the term photoresist is used hereinafter to denote the radiation-sensitive polymers and their pre-polymers.
The high cost of the photoresist pre-polymer solutions makes it desirable to devise methods of improving the efficiency of the coating process so as to minimize the amount of the polymer solution required to coat a substrate. Furthermore, thickness uniformity of the photoresist layer is an important criterion in the manufacture of integrated circuits. When the radiation is focused through the mask onto the coating, variations in thickness of the coating prevent the precise focus over the entire surface of the wafer which is required to obtain the sharpness necessary to ensure satisfactory reproduction of the geometric patterns on the semiconductor wafer for advanced circuits with line width dimensions approaching 0.25 micron line widths and smaller over a surface.
The solvent in the photoresist tends to evaporate during application, increasing the viscosity of the polymer solution and inhibiting the leveling of the resulting film. This produces thickness non-uniformities. It is therefore desirable to be able to control the rate of evaporation of solvent from the polymer solution during the coating process.